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Re: How to handle Units of Measure
*To*: mathgroup at smc.vnet.net
*Subject*: [mg100239] Re: How to handle Units of Measure
*From*: AES <siegman at stanford.edu>
*Date*: Thu, 28 May 2009 19:35:39 -0400 (EDT)
*Organization*: Stanford University
*References*: <gvlhom$djb$1@smc.vnet.net>
In article <gvlhom$djb$1 at smc.vnet.net>,
Alexei Boulbitch <Alexei.Boulbitch at iee.lu> wrote:
> In this very simple example if I write
> R+iwL
> where R is measured in Ohms, w in s^-1, L in H, I expect that the
> result is given in Ohms.
> Why it doesn't happen in Mathematica ? Ho can I do to handle in a
> simple way those conversions ?
> Thanks
Here is one way to handle -- and think about -- these situations, which
works for me.
Your example involves the equation
v = (r + j w l) i
which is a true equation provided _all_ the quantities in it are in a
_consistent_ set of units (volts, ohms, radian/sec, henries, and amps
would be are a consistent and widely used set for this case).
So, in _any_ calculation you do, always write *all the equations that
are going to be used to do the calculations* in a consistent set of
units (mks is good, but you might for some reason want to use another
set: cgs, ipj, etc).
But, you might also want to provide _input_ data to these calculations,
or print or plot _output_ data from these calculations, using other
units. As just one example, you might want to do calculations with the
above equation using the standard units given the above, but supply
input date for r in kilo-ohms and display output results for i in
milliamps.
To do this, define at some appropriate spot early in your calculations
the "units"
kOhm = 10^3; mA = 10^-3; (* use whatever names you like *)
If you want to put into the calculation at some point an input
resistance of 10 kilohms, or an input current of 10 milliamps, you
_input_ these values as
r = 10 kOhm; OR i = 10 mA; (* for _inputs_ *)
which means the numerical values of r and i in the calculations are now
in the correct basic set of units.
If on the other hand, at some later point you want to, say, Plot or
tabulate an output current in milliamps (remembering that it is
_calculated_ in amps). or a resistance in kilohms, you write
Plot[ i/mA, { . . . }] OR Table[ r/kOhm, { . . . }]
The mental construct to keep in mind in this latter case is that writing
"i/mA" in Mathematica now gives you the value of the current i _in
milliamps_.
This approach doesn't put _labels_ of any sort on these outputs; but you
should preferably handle those labels yourself anyway, and keep them
separate from the calculations.
In the above case, for example, you can put "r in kilohms" in the column
heading of the Table, or write
Print[ "Current = "<>ToString[i/mA]<>" milliamps"]
or
Print[ "Current in milliamps = "<>ToString[i/mA]]
to label the plot.
I find this approach, used consistently, to be consistent and readable
in all my notebooks.
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